Implantable microstimulator and systems employing the same

ABSTRACT

Improved implantable microstimulators covered with a biocompatible polymeric coating in order to provide increased strength to the capsule thereof and to capture fragments of the microstimulator should it become mechanically disrupted and to make the microstimulator safer and easier to handle are provided here. The coating may include one or more diffusible chemical agents that are released in a controlled manner into the surrounding tissue. The chemical agents, such as trophic factors, antibiotics, hormones, neurotransmitters and other pharmaceutical substances, are selected to produce desired physiological effects, to aid, support or to supplement the effects of the electrical stimulation. Further, provided herein are systems employing the improved microstimulators to prevent and/or treat various disorders associated with prolonged inactivity, confinement or immobilization of one or more muscles. Such disorders include pressure ulcers, venous emboli, autonomic dysreflexia, sensorimotor spasticity and muscle atrophy. These systems include external control means for controlling the operation of the microstimulators, said control means having memory means for programming preferred stimulation patterns for later activation by the patient or caregiver.

This application claims the benefit of provisional application Nos.60/011,870 filed Feb. 20, 1996, 60/012,019 filed Feb. 20, 1996,60/011,868 filed Feb. 20, 1996 and 60/011,869 filed Feb. 20, 1996.

BACKGROUND OF THE INVENTION

Muscles serve a number of functions, most of which are dependent upontheir regular contraction, which is in turn dependent upon theirstrength and health. For example, in addition to the well knownfunctions of supporting the skeleton and permitting movement, musclesserve to pad the force of bone protuberances against the skin, and theypromote blood flow, particularly through deep blood vessels. In responseto repeated contractions against a load, muscle fibers grow incross-sectional area and develop more force, and in response to repeatedcontraction over a long period of time, the oxidative capacity and bloodsupply of the fibers is enhanced.

In normal individuals, muscles are activated to contract by electricalsignals that are communicated from the brain and spinal cord by way ofmuscle nerves. Many medical diseases, physical disabilities and cosmeticdisfigurements arise from abnormal or absent electrical signals to themuscles. Such abnormal or absent electrical signals may be pathologicalor may simply be due to prolonged immobility or confinement thatrestricts or prevents the voluntary movement of one or more muscles.Without normal, routine electrical stimulation, muscles atrophy, that islose their normal size and strength. Also contributing to muscle atrophymay be a wide range of other pathophysiological mechanisms, includingabsence of sustaining hormones and other endogenous trophic substances.

Many situations exist in which voluntary muscle contraction cannot beused effectively to operate, condition or strengthen muscles. The mostextreme loss of voluntary muscle function occurs when the brain orspinal cord is injured by trauma, the growth of tumors orcerebrovascular accidents. In patients suffering from these conditions,muscles become wholly or partially paralyzed because the electricalcommands that are normally generated in the nervous system are no longeravailable to stimulate muscle contractions. Less extreme degrees ofmuscle weakness and atrophy can come about when some of the nerve fiberssupplying a muscle are damaged by disease or injury, or when the muscleis immobilized or voluntarily rested, for example by casting or bedrest,in order to recover from an injury or surgical intervention involving anearby body part, or other prolonged confinement or immobilization.

With respect to prolonged physical confinement or immobilization, theaffect of muscle non-use and atrophy frequently leads to two disordersthat are particularly difficult to avoid and expensive to treat,pressure ulcers of the skin and subcutaneous tissues and retardation ofthe normal circulation of blood through deep vessels. Continual,unrelieved pressure on localized regions of skin can result in thedevelopment of pressure ulcers of the skin and subcutaneous tissues,also known as bed sores or decubitus ulcers. Pressure ulcers are thoughtto occur when tissues underlying a site of pressure are deprived ofoxygen and nutrients because blood flow is impeded, and when the area issubjected to frictional and shearing forces associated with continuousrubbing and movement. Pressure ulcers vary in size and degree of damagefrom small regions of redness to deep craters of tissue erosion passingthrough skin, connective tissues, muscle and even bone that can threatenthe life of a patient by providing portals of entry for pathogenicorganisms. They are often exacerbated in chronically paralyzed orbedridden patients because of atrophy of the unused muscles thatnormally provide a degree of padding between the skin and underlyingbony protuberances. The treatment of pressure ulcers often requiresprolonged, intensive medical care and occasionally extensive surgery,usually entailing further restrictions in the posture of the patient,which may further complicate medical and nursing care and cause othercomplications.

As mentioned above, prolonged immobilization or physical confinement ofa body part often also results in retardation of circulation of bloodthrough deep vessels, particularly the veins in an around muscles. Forexample, the failure to contract muscles in the limbs at regularintervals, as occurs normally when walking or standing, is known tocause stasis of blood in some veins. Venous stasis is a predisposingfactor in the formation of clots in the veins. Such deep venousthrombosis further compromises blood flow to the immobilized body partand can be the source of dangerous emboli to the heart and lungs.Thrombosed veins may also become chronically infected, posing a dangerof septicemia. Examples of particular populations of patients that areespecially at risk for development of pressure ulcers and venous emboliinclude comatose and obtunded patients, patients who are confined byparalysis to bed or wheelchairs, bedridden patients who have medical orsurgical conditions that limit their activity, and elderly patients withlimited mobility. To reduce complications in these patients, it isnecessary to reestablish movement of the vulnerable body parts; however,these patients are either incapable of voluntary movement or severelyrestricted in their ability to voluntarily move. Therefore, therapistsoften spend considerable time manipulating the passive limbs of thesepatients, but this is expensive and relatively ineffectual because it isthe active contraction of muscle that tends to pump blood through theveins and to maintain the bulk of the muscle.

It has long been known that muscle contractions can be elicitedinvoluntarily by stimulating muscles and their associated motor nervesby means of electrical currents generated from electronic devices calledstimulators. This has given rise to various therapies that seek toprevent or reverse muscle atrophy and its associated disorders by theapplication of electrical stimulation to the muscles and their nervesvia these stimulators. For example, the field of research known asfunctional neuromuscular stimulation (FNS) or functional electricalstimulation (FES) has begun, which seeds to design and implement devicescapable of applying electrical currents in order to restore functionalmovement to paralyzed limbs. Similarly, therapies employing stimulatorsto regularly apply specific patterns of electrical stimulation tomuscles in order to prevent or reverse atrophy are known.

Many of the earliest stimulators were bulky and relied upon the deliveryof large current pulses through electrodes affixed to the skin, aprocedure that requires careful positioning and fixation of theelectrodes to the skin and frequently produces disagreeable cutaneoussensations and irritation of the skin. Additionally, such transcutaneousstimulation produces relatively poor control over specific muscles,particularly those that lie deep in the body. Thus, this procedure canbe time-consuming, uncomfortable, and is generally useful only formuscles located immediately beneath the skin.

It is also possible to stimulate muscles more directly by passingelectrodes through the skin into the muscles or by surgically implantingself-contained stimulators and their associated leads and electrodes inthe body. These devices have many configurations, but most are large andhave numerous leads that must be implanted and routed through the bodyto the desired muscles using complex surgical methods. Further, they areexpensive to produce and the invasive procedures required for theirimplantation are impractical for most patients because they increaserather than decrease the required care and the danger of infection andother sources of morbidity in patients who are already seriously ill.Thus, such devices have been used primarily in patients with severeparalysis in order to demonstrate the feasibility of producingpurposeful movements such as those required for locomotion, hand-graspor respiration.

More recently a new technology has been described whereby electricalsignals can be generated within specific tissues by means of a miniatureimplanted capsule, referred to as a "microstimulator", that receivespower and control signals by inductive coupling of magnetic fieldsgenerated by an extracorporeal antenna rather than requiring anyelectrical leads. See, U.S. Pat. Nos. 5,193,539; 5,193,540; 5,324,316;and 5,405,367, each of which is incorporated in its entirety byreference herein. These microstimulators are particularly advantageousbecause they can be manufactured inexpensively and can be implantednon-surgically by injection. Additionally, each implantedmicrostimulator can be commanded, at will, to produce a well-localizedelectrical current pulse of a prescribed magnitude, duration and/orrepetition rate sufficient to cause a smoothly graded contraction of themuscle in which the microstimulator is implanted. Further, operation ofmore than one microstimulator can be coordinated to provide simultaneousor successive stimulation of large numbers of muscles, even over longperiods of time.

While originally designed to reanimate muscles so that they could carryout purposeful movements, such as locomotion, the low cost, simplicity,safety and ease of implantation of these microstimulators suggests thatthey may additionally be used to conduct a broader range of therapies inwhich increased muscle strength, increased muscle fatigue resistanceand/or increased muscle physical bulk are desirable; such as therapiesdirected to those muscle disorders described above. For example,electrical stimulation of an immobilized muscle in a casted limb may beused to elicit isometric muscle contractions that would prevent theatrophy of the muscle for the duration of the casting period andfacilitate the subsequent rehabilitative process after the cast isremoved. Similarly, repeated activation of microstimulators injectedinto the shoulder muscles of patients suffering from stroke would enablethe paretic muscles to retain or develop bulk and tone, thus helping tooffset the tendency for such patients to develop subluxation at theshoulder joint. Use of microstimulators to condition perineal muscles asset forth in applicants' copending patent application, Ser. No.08/007,521, filed Nov. 24, 1995, entitled "Method for ConditioningPelvic Musculature Using an RF-Controlled Implanted Microstimulator",incorporated herein by reference, increases the bulk and strength of themusculature in order to maximize its ability to prevent urinary or fecalincontinence.

In addition to the therapeutic use of microstimulators to promotecontraction of specific, isolated muscles in order to prevent or remedythe disorders caused or contributed to by inactive muscles, theadministration of hormones, trophic factors and similar physiologicallyactive compounds may also be useful. It is known that the extent towhich a muscle will grow in response to any stimulation regime isaffected by the hormonal and chemical environment around the muscle.Muscle fibers have receptors for many physiologically active compoundsthat circulate normally in the blood stream or are released from nerveendings. These trophic factors have significant effects on the nature,rate, and amount of growth and adaptation that can be expected of themuscle in response to stimulation, whether such is produced voluntarilyor by electrical stimulation. Perhaps the best-known of these hormonesare the androgenic steroids often used by athletes to increase musclebulk and strength; but other hormones such as estrogens and growthhormones are also known to affect muscle properties. For example, thedramatic reductions in circulating estrogens and androgens that occur inwomen following menopause appear to account for decreases in the mass ofmuscles and bones, which can be slowed or even reversed by administeringthe deficient hormones systemically.

Thus, the beneficial strengthening effects of electrical stimulation canbe maximized by providing the affected muscles with a supportivehormonal environment for growth. These compounds can be providedsystemically by administering them orally or by injection. However, manysuch compounds are rapidly metabolized by the liver, so that high dosesmust be administered to achieve a desirable therapeutic effect. This canexpose all tissues of the body, including the liver, to high and perhapspoorly controlled levels of the compound, resulting in undesirableside-effects that may outweigh the desired actions of the agent. In oneaspect, the present invention recognizes that this problem could becircumvented by using a more selective method of drug delivery directedspecifically to the electrically exercised muscles. Even if theintroduced compound were ultimately to be cleared by absorption into thebloodstream, high concentrations would be produced only in the tissuearound the target. A steep dilutional gradient would ensure that otherregions of the body were exposed to much lower levels of theadministered compound. By providing a more conducive chemicalenvironment in the early stages of electrical therapy, it is expectedthat muscle atrophy could be reversed more rapidly and effectively.After muscle function has been reestablished, longer-term performance ofthe muscle could be more easily maintained at the desired level byelectrical stimulation alone or in combination with low-dose systemicreplacement therapy.

The microstimulators described and claimed herein are elongated deviceswith metallic electrodes at each end that deliver electrical current tothe immediately surrounding biological tissues. The microelectroniccircuitry and inductive coils that control the electrical currentapplied to the electrodes are protected from the body fluids by ahermetically sealed capsule. This capsule is typically made of a rigiddielectric material, such as glass or ceramic, that transmits magneticfields but is impermeable to water vapor.

Encapsulation in glass is an effective and inexpensive way to ensure ahermetic seal between the electronic components and the biologicaltissues. Methods for forming similar hermetic seals within the confineddimensions of the overall device are well-known in the fabrication ofindustrial magnetic reed relays and diodes and have been describedspecifically for implantable microstimulators. See, e.g., U.S. Pat. Nos.4,991,582; 5,312,439; and 5,405,367, each of which is incorporated inits entirety by reference herein. Such a hermetic barrier is importantboth to ensure good biocompatibility with the body and to protect thesensitive electronics from the body fluids that might destroy theirfunction.

Unfortunately, however, glass and similarly brittle materials such asceramic may crack or shatter as a result of externally applied forces oreven residual stress in the crystalline structure of the materialitself. If such an event occurs within the body or during a surgicalprocedure, it is desirable to retain or capture the sharp fragments ofthe capsule and any internal components so that they do not irritate ormigrate into the surrounding tissues. In a testing or surgicalenvironment in which devices are handled repeatedly, the hard, slipperysurface of the glass capsule makes the device difficult to handle, andcould increase the likelihood that the device will be dropped or pinchedwith a force sufficient to break the glass. Therefore, in one aspect,the present invention provides a well-chosen biocompatible coating forthe glass which would decrease the lubricity of the device and ensurethat glass pieces resulting from device fracture would becontained/captured in a protective sleeve.

The reaction of a living body to an intact foreign body such as animplanted microstimulator depends at least in part on the shape andtexture of the surface of the foreign body, as described, e.g., byWoodward and Salthouse (1986). The surfaces left by the manufacturingprocesses used for the implanted microstimulator are constrained by thenature of the materials and processes required to achieve the desiredelectronic and mechanical characteristics of the device. Therefore,modification of the microstimulators' chemical nature and/or superficialphysical contours to avoid, prevent and/or discourage an immunologicalresponse by the body, would be advantageous. Additionally, in selectingan appropriate coating material the opportunity arises for theintroduction of various chemical compounds, such as trophic factorsand/or hormones, as discussed above, into or onto the coating. Suchcompounds could then diffuse from the surface of the coating into thesurrounding tissues for various therapeutic and diagnostic purposes, aspreviously mentioned.

SUMMARY OF THE INVENTION

The present invention provides for the prevention and treatment ofvarious disorders caused or exacerbated by abnormal or absent electricalsignals to the muscles and apparatus useful therefore. In one aspect,the invention provides an improved microstimulator having abiocompatible polymeric coating on portions of its exterior, therebyreinforcing the mechanical strength of the microstimulator such that itmay optionally be implanted deeply into the muscle, while also providinga means for capturing the fragments of the microstimulator shouldmechanical disruption occur. In preferred embodiments, the coatingsprovided herein are selected to improve the nature of the foreign bodyreaction to the implanted microstimulator by modifying its chemicalsurface, texture and/or shape.

The implantable microstimulators disclosed and claimed herein arepreferably of a size and shape that allows them to be implanted byexpulsion through a hypodermic needle or similar injectable cannula. Themicrostimulator includes a hermetically-sealed housing, at least twoexposed electrodes, and electronic means within the housing forgenerating an electrical current and applying the electrical current tothe exposed electrodes. The coating, as described in detail herein, isformed on at least a portion of the exterior of the microstimulator incontact with the hermetic seal.

In another aspect of the present invention, the improvedmicrostimulator, in addition to providing electrical stimulation to themuscle within which it is implanted, is modified to provide a locallyhigh level of one or more desired chemical agents or drugs. In apreferred embodiment, the polymeric coating covering a portion of themicrostimulator's surface contains a chemical agent that is releasedgradually from said coating. Thus, when the microstimulator is implantedwithin or adjacent to a muscle it produces an electrical current thatactivates the motor nerves and/or muscle fibers of the muscle whilesimultaneously dispensing the chemical agent(s) in the vicinity of theactive muscle fibers.

Further, in preferred embodiments, the improved microstimulator isdesigned to provide electrical stimulation over a period of many yearsand to provide elution of the chemical agent(s) over a period of manydays, weeks or longer without any percutaneous connections to theexternal world. Release of the chemical agent from the coating of themicrostimulator may be by diffusion or, alternatively, may be at apredefined rate controlled by electrical signals produced by theimplantable device.

In yet another aspect of the present invention, systems providinginvoluntary movement to muscles for the purpose of preventing, treatingand/or slowing the progress of various complications associated withmuscle inactivity, especially inactivity due to prolonged physicalconfinement or immobilization, are provided. These systems employ one ormore microstimulators non-surgically implanted in or near one or moreinactive muscles. Once implanted, the prescribing physician uses anexternal controller to command each of the implanted microstimulators toproduce various output stimulation pulses in order to determine apattern of stimulation that produces the desired muscle contractionpattern. The external controller retains the programmed stimulationroutine and, thereafter, administers the therapy on a regularlyscheduled basis and/or whenever commanded to do so by the patient or anycaregiver.

The systems provided herein are particularly useful for maintaining orimproving the functional capacity of paralyzed, weak, immobilized orunderexercised muscle without requiring voluntary exercise and forpreventing various complications of prolonged physical confinement,including but not limited to pressure ulcers, deep venous thrombosis,autonomic dysreflexia and sensorimortor spasticity. For example, theimplantable microstimulators are employed to stimulate specific musclesin order to reduce the incidence and accelerate the healing of pressureulcers on the sacrum, heels and other bony protuberances of bedridden orimmobilized patients. Alternatively or additionally, the systems areemployed to reduce the possibility of venous stasis and embolusformation by eliciting regular muscle contractions in the legs of thebedridden or otherwise immobilized patient. Advantageously, thesesystems may be employed to produce the desired pattern of regularcontractions in one or more muscles for periods of days or weeks withoutthe need for ongoing, continuous patient or caregiver supervision.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the presentinvention will be more apparent from the following more particulardescription thereof, presented in conjunction with the followingdrawings wherein:

FIG. 1 diagrammatically illustrates one embodiment of a microstimulatorcoated with a polymeric coating in accordance with the presentinvention;

FIG. 2 shows another embodiment of a microstimulator coated with apolymeric coating wherein the coating extends over a portion of theelectrodes;

FIG. 3 illustrates another embodiment of the implantable device of thepresent invention that provides both electrical stimulation and releaseof a chemical agent;

FIG. 4 shows another embodiment of an implantable device in accordancewith the present invention;

FIG. 5 diagrammatically illustrates an implanted microstimulator inmuscle tissue and its control using an external controller;

FIG. 6 shows a variation of the invention wherein a battery is includedwithin the implantable device to allow it to operate independently ofthe external controller;

FIG. 7 illustrates a preffered manner used to implant a microstimulatorin accordance with the present invention;

FIG. 8 illustrates the general circumstances that give rise to pressureulcers, and illustrates one preferred manner in which an implantedmicrostimulator, in accordance with the present invention, may be usedto reduce pressure ulcer formation; and

FIG. 9 illustrates the general circumstances that give rise to venousstasis, and further shows a preferred manner in which an implantedmicrostimulator may be used and controlled, in accordance with thepresent invention, to prevent and treat such a condition.

Corresponding reference characters indicate corresponding componentsthroughout the views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best mode presently contemplated forcarrying out the invention. This description is not to be taken in alimiting sense, but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe determined with reference to the claims.

An implantable device 9 made in accordance with the present invention isillustrated in FIG. 1. The device 9 includes a narrow, elongated capsule2 containing electronic circuitry 4 connected to electrodes 6 and 8,which pass through the walls of the capsule at either end, togetherforming a microstimulator of the type disclosed and fully described inU.S. Pat. Nos. 5,193,539; 5,193,540; 5,324,316 and 5,405,367, each ofwhich is incorporated herein, in its entirety, by reference. A coating10 is applied over the longitudinal extent of the surface of the capsule2. In the particular embodiment of FIG. 1, the ends of the coating 11 donot extend over the surface of electrodes 6, 8, so that the coating doesnot change the overall profile of the microstimulator. The device 9 isshaped to permit its insertion through a tubular insertion cannula, suchas a syringe, that can be passed transcutaneously into a target musclewith or without fluoroscopic guidance, as described further below.

The capsule 2 may be made of glass or a similar dielectric material,such as ceramic, that can provide a hermetic barrier to the permeationof body fluids and water vapor into circuitry 4. The basic design of thecurrent-generating circuitry 4 is the same or similar to that describedin the above-referenced patents, in which electrodes 6 and 8 may becontinuously charged (through inductive coupling) by a programmablemagnitude of direct current and may be occasionally discharged so as toproduce a large, brief stimulation pulse with a programmable magnitudeand duration, which stimulation pulse is used for the activation ofnearby motor nerve and/or muscle fibers.

The coating 10 of the improved microstimulator is selected to both bebiocompatible and to be elastic enough to provide some reinforcement tothe capsule 2. Additionally, it is advantageous and preferred that thematerial chosen to form the coating 10 serve to reduce the risk ofinjury from and to provide means for the capture of capsule fragments inthe event the capsule is broken. Finally, it is desirable that thecoating 10 chosen reduces the lubricity of the device, as glass andceramic materials, of which the capsule 2 is most often constructed, areslippery. It will be appreciated by those of skill in the art thatseveral different coatings are available having these characteristics.By way of example only and in no way to be limiting, the polymericcoating 10 may be formed of a silicone elastomer or a thermoplasticmaterial, such as polyethylene, polyester, polyurethane or a fluorinatedcarbon chain from the TEFLON family.

The preferred method of application of coating 10 depends on itschemical composition and physical properties. For example, in oneembodiment, the coating 10 is formed from a thin-walled extrusion ofsilicone elastomer whose inside diameter is slightly smaller than theoutside diameter of capsule 2. The extruded tubing is cut to the desiredlength and its diameter temporarily expanded by absorption of anappropriate solvent such as heptane, toluene or xylene. The expandedsilicone tubing is then slipped over the microstimulator, subsequentlyshrinking tightly onto the surface of the microstimulator as the solventis evaporated from the silicone elastomer, thereby forming the desiredcoating 10.

In another embodiment, the coating 10 is made from a thermoplasticmaterial such as a polyethylene, polyester, polyurethane or afluorinated carbon chain from the TEFLON family. A thin-walled extrusionof said thermoplastic material whose inside diameter is smaller than theoutside diameter of capsule 2 is mechanically expanded so as totemporarily increase its inside diameter. The expanded extrusion is thencut to the desired length, slipped over the microstimulator, and causedto shrink onto the surface of the microstimulator by briefly heating itto the temperature at which it contracts toward its unexpandeddimensions, thereby forming the desired coating 10.

In another embodiment, coating 10 is made from a liquid solutioncontaining melted, dissolved or unpolymerized material which is appliedto the surface of the microstimulator by dip-coating, injection molding,or other suitable methods known to those of skill in the coating art.After covering the desired portions of the microstimulator, the coating10 is allowed or caused to harden by appropriate means.

FIG. 2 shows an alternative embodiment of a microstimulator 16 inaccordance with the present invention. The microstimulator 16 of FIG. 2is similar to the microstimulator 9 of FIG. 1 except that in FIG. 2 theends 12 of coating 10 extend over electrodes 6, 8, thereby preventingconcavities 14 from coming into direct contact with tissues surroundingthe implanted microstimulator. Advantageously, concavities 14 may befilled with a solid material, such as silicone or other material, toeliminate the presence of pockets of fluid that may act as a nidus ofchronic infection.

FIG. 3 shows another embodiment of an improved microstimulator 18 inaccordance with the present invention. The microstimulator 18 of FIG. 3is similar to the microstimulator 9 of FIG. 1 except that the coating 10in FIG. 3 contains a chemical agent 20 which diffuses from the surfaceof the coating 10 into the surrounding tissues. The chemical agent 20may be any of a large number of pharmacologic and diagnostic agentswhose presence in the tissue surrounding the implantable microstimulatoris desired as part of the treatment received by the patient. Examples ofsuitable chemical agents 20 include anti-inflammatory or antibioticcompounds intended to reduce the foreign body reaction, hormones,neuromodulators and neurotransmitters intended to potentiate the effectsof the electrical currents, or dyes intended to mark the originallocation of the implanted microstimulator. This list of agents providesonly examples and is not intended to limit the scope of the inventionset forth in the claims.

The method of introduction of the chemical agent 20 into or onto thecoating 10 depends upon the chemical nature of the agent and theselection of an appropriate coating material. In general, the types ofagents and compatible coatings that may be used therewith are known tothose of skill in the arts of chemical binding and diffusion and thedesign of sustained release pharmaceuticals.

In a preferred embodiment, the chemical agent 20 comprises a long-actingcompounds of testosterone, such as testosterone propionate, cypionate orenanthate. This agent 20 is mixed with or adsorbed onto a siliconeelastomer that is injection-molded or dip-coated and subsequentlypolymerized to provide a thin coating 10, which coating 10 is spreadover a substantial portion of the surface area of the capsule 2. Itshould be appreciated that silicone is a highly biocompatible compoundthat has been used previously to administer steroids to experimentalanimals without exposing the animals to the trauma of repeatedinjections. However, it should also be appreciated that coating 10 couldbe formed from a variety of other materials, or by using a variety ofother processes, as described above.

It is thus seen that in this preferred embodiment, agent 20 comprises atrophic compound used to enhance muscle development, specifically atestosterone derivative. It should be appreciated that such compoundshave been used for many years in humans to treat endocrine disorders orto retard the development of estrogen-sensitive mammary tumors, and thata single intramuscular bolus of the compound will exert its actions for2 to 4 weeks. The chemical agent 20 associated with the external coating10 of the present invention, however, could be selected from a varietyof trophic chemicals with actions on muscle or connective tissues, andcould be bound to the coating in any manner that advantageously affectsits rate of release. The rate of release may be designed to be anywherefrom a few hours to a few days or weeks. Furthermore, agent 20 mightactually consist of a multiplicity of active compounds, various of whichaffect or influence muscle fibers, nerve fibers, connective tissue, orinflammatory cells so as to modify many aspects of the response of thetissues to the presence and activation of the device.

Certain composite materials, such as the drug-filled polymeric matrixthat may be used for coating 10 the device, have the property thatelectrical voltage influences a change in the rate at which the fillersdiffuse form the matrix. Where it is desirable to use such compositions,the microstimulator illustrated in FIG. 2 is particularly useful, as theelectrical output signals generated by circuit 4 are applied, at leastin part, to the coating 10 by its contact with the electrodes 6, 8 ofthe device. Such electrical output signals are systematically varied soas to produce the desired rate of elution of the chemical agent 20 intothe tissues surrounding the implanted device. Thus, it is seen that theelectrical currents produced by electrodes 6 and 8 in the process ofstimulating the muscle could also advantageously have the effect ofincreasing the elution rate of agent 20 simultaneously with theelectrically-induced muscle contraction.

As illustrated in FIG. 4, rate control of the elution of the chemicalagent 20 from the coating 10 may alternatively be managed usingadditional electrodes 26 which are affixed to the capsule 2 andconnected to the circuitry 4 of the device. Such additional electrodesprovide for separate control of the electrical currents and voltagesapplied to stimulate the muscle electrically and to control the rate ofelution of chemical agent 20 from the polymeric coating 10.Advantageously, such multiple electrodes facilitate the use ofelectrophoretic current through coating 10 to effect the release ofagent 20, independent of the currents required to charge and dischargethose electrodes associated with muscle or nerve stimulation. Asillustrated in FIG. 4, electrode 26 is entirely covered by the polymericcoating 10, whereas electrodes 6 and 8 are exposed to the body fluids.Electrical current applied between electrodes 26 and 8 would passthrough coating 10 to effect electrophoretic release of chemical agent20. Electrical current applied between electrodes 6 and 8, on the otherhand, would pass unobstructed through the body fluids and tissues toeffect electrical stimulation of nearby nerve or muscle fibers.

Referring now to FIG. 5, an improved microstimulator 28 is shownimplanted into muscle 30. In this embodiment, as well as in those ofFIGS. 1-3, the improved microstimulator receives power form an externalcontrol device 40. The external control device 40 generates analternating magnetic field, illustrated symbolically by the lines 36,through an external coil 38, which coil may advantageously be locatedunderneath the patient in a seat or mattress pad or in a garment or itemof bedclothes. The magnetic field 36 is coupled with an implanted coil33, which forms part of the microstimulator device 28, and induces avoltage and current within the coil 33. The induced voltage/current inthe coil 33 is used to power the electronic circuitry 4, andfluctuations (e.g., modulation) of the varying magnetic field 36 areused to control operation of the electronic circuitry 4. That is, thedevice 28 delivers current to its electrodes 6, 8 according toinstructions encoded in fluctuations of the magnetic field 36.

In this preferred embodiment, electrical current emitted from electrodes6 and 8 stimulates motor nerve fibers 32. Muscle fibers themselves arerelatively difficult to activate via such electrical currents, but themotor nerve fibers are more readily stimulated, particularly if themicrostimulator is located near them in the muscle. Each time a motornerve fiber is excited, it conveys an electrical impulse through itshighly branched structure to synaptic endings on a large number ofmuscle fibers, which results in the activation of essentially all ofthose muscle fibers. Electronic circuit 4, then, controls the amplitudeand duration of the electrical current pulse emitted by themicrostimulator 28, thereby determining the number of such motor nervefibers that are excited by each pulse.

As an example of a preferred use of the improved microstimulator, theprescribing physician uses a programming station 44 to command externalcontroller 40 to produce various stimulation pulses, during the initialtreatment session after implantation of the improved microstimulator 28.This is done in order to determine an exercise program that will providethe desired therapeutic muscle contraction program for the individualpatient. The exercise program is down-loaded into a memory element 42 ofthe external controller 40, where it can be reinitiated at will by, forexample, manually activating control 46. This manual control may beperformed, e.g., by the patient or an attending caregiver. In thepreferred embodiment, programming station 44 is a personal computer,external controller 40 contains a microprocessor, and memory element 42is a nonvolatile memory bank such as an electrically programmableread-only memory (EEPROM). However, it will be appreciated by those ofskill in the art that many different systems, architectures andcomponents can achieve a similar function.

In accordance with a variation of the invention, shown in FIG. 6, abattery 46 is included within the implanted device (microstimulator),and is employed as a continuous source of power for the electroniccircuit 4. Such battery also provides storage and production means for aprogram of output currents and stimulation pulses that may then beproduced autonomously by the implanted device without requiring thecontinuous presence of extracorporeal electronic components, i.e.,without the need for an external control device 40. In such instance,means would be provided for transmitting the desired program to eachmicrostimulator and for commanding each microstimulator to begin or tocease operating autonomously. Advantageously, such an embodiment asshown in FIG. 6 may provide for continuous biasing current or voltageapplied to coating 10 (when at least one of the electrodes is positionedto contact the coating 10, as shown in FIG. 2 above, or when a separateelectrode is embedded in the coating 10 as illustrated in FIG. 4, above)so that the rate of elution of agent 20 would always be well-controlled.

In a preferred implantation method, the microstimulator is injected intothe muscle of interest through an insertion device whose preferredembodiment is shown in FIG. 7. The external cannula 110 of the insertiontool is comprised of a rigid, dielectric material with sufficientlubricity to permit the easy passage of the microstimulator withoutscratching its external surface. The central trochar 120 of theinsertion tool is an electrically conductive rod whose sharpened pointextends beyond the insertion cannula, where it can be used to delivercurrent pulses to the biological tissue near its point. The initialinsertion of the tool is directed either by a knowledge ofmusculoskeletal landmarks or radiographic imaging methods to approachthe region of muscle 30 in which motor nerve fibers 32 enter. Optimally,the insertion device is advanced into the muscle in parallel with thelong axis of muscle fiber fascicles. Electrical stimuli may be deliveredthrough the metallic trochar 120 by connecting a conventional electricalstimulator (not shown) to connector 122 on the trochar. By observing thecontractions of the muscle 30, these test stimuli can be used to ensurethat the tip of the insertion device is situated sufficiently close tomotor nerve fibers 32 to permit activation of a substantial portion ofthe muscle 30 without undesirable activation of other muscles or nerves.Failure to elicit the desired muscle contractions would suggest a poorsite of placement for the microstimulator and a need to reposition theinsertion tool closer to the site of motor nerve entry.

When the desired position is reached, the trochar 120 is removed fromcannula 110, taking care to keep the cannula 110 in position withinmuscle 30, and a microstimulator is pushed through cannula 110 and intomuscle 30 using a blunt-ended push-rod 130.

As stated above, the microstimulators provided herein are particularlyuseful in the prevention and treatment of various disorders associatedwith prolonged immobilization or confinement; such as muscle atrophy,pressure ulcers and venous emboli. Referring to FIG. 8, there isillustrated, in diagrammatic form, the general circumstances that giverise to pressure ulcers and a preferred embodiment whereby one or moremicrostimulators may be employed to reduce the incidence of and/orcontribute to the healing of such pressure ulcers. As depicted in FIG.8, force 52 applied between bone 50 and firm support surface 56 istransmitted through intervening soft tissues of the skin 34 and muscle30, resulting in compression of skin region 54. Skin region 54 is thusin danger of developing a pressure ulcer. Active contraction of muscle30 is induced by electrical stimulation applied by microstimulator 48and its associated electrodes 6 and 8. Such active contraction makesmuscle 30 stiffer, causing force 52 to be dissipated over a largerregion of the skin 34. Further, active contraction of muscle 30 tends toshift the position of the body with respect to surface 56, causing force52 to be directed to a fresh region of skin 34. Regular activecontraction of muscle 30 induces various trophic mechanisms in themuscle that maintain or even enhance the bulk and tone of muscle 30 inits passive state, thereby reducing the concentration of force 52 onskin region 54.

To further aid in the prevention and/or treatment of the pressure ulcer,the microstimulator as described above and illustrated in FIGS. 3-6employing a coating 10 having a chemical agent 20 associated therewith,may be used. In this alternative, the chemical agent 20 may be a trophicfactor employed to improve the bulk and tone of the muscle 30 or may bean antibiotic or similar therapeutic drug useful for preventinginfection of the pressure ulcer, or the chemical agent may be acombination of the two different agents. Increasing the bulk and tone ofthe muscle 30, can provide additional padding between the bone 50 andsupport surface 56, thereby lessening the force 52 against the skinregion 54.

In the embodiment illustrated in FIG. 8, a microstimulator 48 has beeninjected into muscle 30 at (or very near) the skin region site 54 wherea potential pressure ulcer may develop. However, it may be satisfactory(or even preferred in some instances) to inject one or moremicrostimulators into adjacent muscles or near various nerves thatcontrol muscle 30 and/or other muscles that can affect the magnitude anddirection of force 52 upon various regions of skin 34.

It should be appreciated that contraction of many different muscles andgroups of muscles tends to lift the prominence of bone 50 so as todistribute the load of the body more evenly across the skin 34, therebyreducing the amount of force 52 applied at a particular skin region 54.optimally, a particular temporal pattern of stimulation applied by oneor more microstimulators generates a sustained contraction of therespective muscles that is maintained for several seconds to permitblood flow into vulnerable tissues. Such is accomplished by theextracorporeal components illustrated in FIG. 5 and described above.Thus, upon an external command or at predetermined intervals, power andcommand signals sent from controller 40 cause the variousmicrostimulators to emit a series of electrical current pulses (i.e., apulse train) at the desired frequency and amplitude sufficient to causethe muscles to lift the body for the duration of the pulse train.

Further movement of the body part typically occurs after cessation ofsuch pulse-train stimulation because of various nervous reflexes orvoluntary movements that are triggered by the concomitant activation ofvarious sensory nerve fibers resulting either from direct electricalstimulation of the sensory fibers or the mechanical consequences of thedirectly stimulated muscle activity. Such triggered movements aregenerally just as important, and may even be more important, than thedirectly stimulated muscle activity caused by themicrostimulator-generated pulse train for shifting body posture.

FIG. 9 illustrates the general circumstances that give rise to venousstasis and a particular embodiment for the use of microstimulators toreduce such stasis. Blood flow 64 in veins 66 running through andbetween muscles 30, 62 depends in part on compressive forces 52 andgeneral metabolic stimulation resulting from the occasional activecontraction of muscles 30 and 64. In the absence of such contractions,flow is reduced, resulting in stasis and an increased likelihood of theformation of clots or thrombi in the veins. One common site for thisproblem is in the calf muscles of the lower leg, which extend the ankle.In accordance with one aspect of the present invention, therefore, oneor more microstimulators are injected into the extensor muscles of theankle, and one or more microstimulators are injected into the flexormuscles of the ankle. The programmed sequence of stimulation stored inmemory bank 42 is used by controller 40 to create the necessarytransmission of power and command signals from coil 38 to cause themicrostimulators injected into the ankle muscles to generate aprescribed stimulation sequence. Ideally, this prescribed sequenceelicits muscle contractions sufficient to shift the position of the footalternately into extension and flexion for several seconds. The intervalbetween the various muscle contractions and the strength and duration ofthe contraction in each muscle is set by an attending physician orphysiotherapist using a programming station 44 that downloads thedesired program into memory bank 42. The rhythmic intermittent musclecontractions produced each time the program is activated causescompressive forces 52 to act on deep veins 66, augmenting venous flow 64out of the muscle by a pumping action that reduces venous stasis.

It should also be noted that a particular pattern of stimulation appliedthrough a particular microstimulator, or combination ofmicrostimulators, may also be effective at reducing the incidence ofboth pressure sores and venous stasis simultaneously, as well asgenerating other useful trophic effects on the muscles themselves,metabolic stimulation of the cardiorespiratory system, and improvementsin the functioning of nervous pathways responsible for various reflexiveand autonomic functions commonly affected adversely by prolongedimmobilization. Other specific dysfunctions that have been reported tobe reduced by regular electrical stimulation of nerves and musclesinclude autonomic dysreflexia and sensorimotor spasticity, particularlyin patients suffering from spinal cord injury.

It should also be noted that the particular complications of pressuresores and venous stasis illustrated respectively in FIGS. 8 and 9 areintended only to provide specific examples of the beneficial effects ofregular, active muscle exercise that can be induced by microstimulators,and are not intended to limit the scope of the invention set forth inthe claims regarding the utility of stimulation applied in this manner.The present invention pertains generally to all beneficial effects thata caregiver might achieve by the appropriate implantation andprogramming of one or more microstimulators in any patient immobilizedfor a period of more than a few days.

While the invention herein disclosed has been described by means ofspecific embodiments and applications thereof, numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the scope of the invention as set forth in the claims.

What is claimed is:
 1. An implantable microstimulator comprisinganhermetically-sealed housing; at least two exposed electrodes extendingfrom said housing; electronic circuitry within said housing coupled tosaid at least two exposed electrodes for generating electrical currentand applying said electrical current to the two exposed electrodes; saidmicrostimulator being of a size and shape capable of implantation byexpulsion through a cannula or hollow needle; and a polymeric coatingthat covers a substantial portion of the surface area of saidhermetically-sealed housing, wherein said polymeric coating furthercontacts and covers a portion of the exposed electrodes.
 2. Theimplantable microstimulator of claim 1 wherein said coating comprises asilicone elastomer.
 3. The implantable microstimulator of claim 1wherein the coating that covers a substantial portion of the surfacearea of the hermetically-sealed housing comprises a thin-walledextrusion of silicone elastomer forming a tube, said tube having aninside diameter that is slightly smaller than the outside diameter ofthe hermetically-sealed housing, which silicone tube is temporarilyexpanded by absorption of an appropriate solvent so that it can befitted over the hermetically-sealed housing.
 4. The implantablemicrostimulator of claim 1 wherein the polymeric coating that covers asubstantial portion of the surface area of the hermetically-sealedhousing is made from a thin-walled tubular extrusion of a thermoplasticmaterial having an inside diameter that is slightly smaller than theoutside diameter of the hermetically-sealed housing, which siliconetubular extrusion is temporarily expanded to slide over thehermetically-sealed housing and then shrunk with heat to tightly fitover the hermetically-sealed housing.
 5. The implantable microstimulatorof claim 4 wherein said thermoplastic material is selected from thegroup consisting of polyethylene, polyester, polyurethane and afluorinated carbon chain.
 6. The implantable microstimulator of claim 1wherein said polymeric coating comprises a liquid containing melted,dissolved or unpolymerized material that is applied to the surface ofthe hermetically-sealed housing in its liquid form and then cured orhardened to a solid form.
 7. The implantable microstimulator of claim 1wherein the at least two exposed electrodes are adapted to stimulatemotor nerve fibers or muscle fibers.
 8. The implantable microstimulatorof claim 1 wherein said polymeric coating includes a chemical agenttherein adapted to diffuse from the surface of the coating intosurrounding tissue.
 9. The implantable microstimulator of claim 8wherein said chemical agent is adapted to improve the manner in whichthe coated microstimulator is received within tissue after it isimplanted.
 10. The implantable microstimulator of claim 8 wherein saidchemical agent comprises a trophic compound adapted to enhance muscleand tissue development.
 11. The implantable microstimulator of claim 10wherein said trophic compound is selected from the group consisting ofsteroids, testosterone, and long-acting compounds of testosterone. 12.The implantable microstimulator of claim 8 wherein said coatingcomprises a polymeric matrix coating filled with the chemical agent, andwherein the rate of release of the chemical agent from the matrixcoating is influenced by an electrical current applied to the exposedelectrodes of the microstimulator, whereby electrical currents appliedto said exposed electrodes modify the rate of release of the chemicalagent from the matrix coating.
 13. The implantable microstimulator ofclaims 1 or 8 wherein said electronic circuitry within saidhermetically-sealed housing is powered and controlled by externalsignals.
 14. The implantable microstimulator of claims 1 or 8 whereinsaid electronic circuitry within said hermetically-sealed housingincludes a battery which allows operation of the electronic circuitry inthe absence of external signals.
 15. A system for the treatment of bodytissue comprising:(a) an implantable microstimulator comprising anhermetically-sealed housing, at least two exposed electrodes external tosaid housing and coupled to electronic circuit means within saidhousing, wherein the electronic circuit means within said housinggenerates electrical signals that are applied to said electrodes, and acoating containing a pharmaceutical agent, wherein said coating covers asubstantial portion of the hermetically-sealed housing; and (b) controlmeans for controlling said implantable microstimulator to electricallyexcite a physiological response from body tissue near the exposedelectrodes simultaneously with elution of a locally high level of thepharmaceutical agent from the coating.
 16. The system of claim 15wherein the coating that covers a substantial portion of thehermetically-sealed housing is applied so that it also contacts at leastone of said exposed electrodes, and wherein electrical signals appliedto said exposed electrodes affect the rate at which the pharmaceuticalagent is eluted from the coating.
 17. An implantable microstimulatorcomprisingan hermetically-sealed housing; at least two exposedelectrodes extending from said housing; electronic circuitry within saidhousing coupled to said at least two exposed electrodes for generatingelectrical current and applying said electrical current to the twoexposed electrodes; said microstimulator being of a size and shapecapable of implantation by expulsion through a cannula or hollow needle;and a polymeric coating that covers a substantial portion of the surfacearea of said hermetically-sealed housing, wherein said polymeric coatingincludes a chemical agent therein adapted to diffuse from the surface ofthe coating into surrounding tissue.
 18. The implantable microstimulatorof claim 17 wherein said coating does not contact said exposedelectrodes.
 19. The implantable microstimulator of claim 17 wherein saidcoating contacts and covers a portion of the exposed electrodes.
 20. Theimplantable microstimulator of claims 17, 18 or 19 wherein saidpolymeric coating comprises a liquid containing melted, dissolved orunpolymerized material that is applied to the surface of thehermetically-sealed housing in a liquid state then cured or hardened toa solid state.
 21. The implantable microstimulator of claims 17, 18 or19 wherein the at least two exposed electrodes are adapted to stimulatemotor nerve fibers or muscle fibers.
 22. The implantable microstimulatorof claim 17 wherein said chemical agent is adapted to improve the mannerin which the coated hermetically-sealed housing is received withintissue after it is implanted.
 23. The implantable microstimulator ofclaim 17 wherein said chemical agent comprises a trophic compound. 24.The implantable microstimulator of claim 23 wherein said trophiccompound is selected from the group consisting of steroids,testosterone, or long-acting compounds of testosterone.
 25. Theimplantable microstimulator of claim 17 wherein said coating comprises apolymeric matrix coating filled with the chemical agent, and wherein therate of release of the chemical agent from the matrix coating isinfluenced by an electric potential applied to the exposed electrodes ofthe microstimulator, whereby electrical potentials applied to saidexposed electrodes modify the rate of release of the chemical agent fromthe matrix coating.
 26. The implantable microstimulator of claim 17including at least three electrodes, wherein at least one of the threeelectrodes is covered by said coating.